The present invention generally relates to motion sensing devices and their fabrication methods. More particularly, this invention relates to a three-axis inertial sensor and a process for its fabrication.
Three-axis inertial sensors that respond to inertial forces in the x, y and z axes are finding applications in the detection of shock, vibration, and acceleration. These sensors have been fabricated with MEMS (micro-electro-mechanical systems) technology using bulk micromachining, surface machining, or a combination of the two. Particular interest in capacitive three-axis accelerometers has existed in part because they are capable of low power consumption and high sensitivity. Such sensors use the displacement of a proof mass in response to input inertial forces (shock, vibration, and/or acceleration) to cause a change in the gap spacing of the proof mass relative to one or more fixed electrodes, thereby producing changes in capacitance between the proof mass and fixed electrodes. Changes in capacitance are detected and used as the input to an interface circuit that converts the capacitance to an electrical signal, such as voltage or frequency.
In the bulk micromachined implementation of a three-axis inertial sensor, the sensor is formed by patterning and etching a device wafer (formed of silicon or another semiconductor material) to define the sensor features including the proof mass, and then bonding the device wafer to another wafer (e.g., silicon or glass) that provides the fixed electrodes for the sensor. While exhibiting high sensitivity, such sensors are relatively large in size. The surface micromachined implementation of a three-axis inertial sensor typically uses deposited films (such as polysilicon) to form the sensor features. Since film thicknesses are typically limited, the sensors can be relatively small, though generally exhibiting lower sensitivity than bulk micromachined sensors due to a smaller proof mass.
Other implementations of three-axis inertial sensors include the use of SOI (silicon on insulator) wafers and epitaxially-deposited silicon (epipoly). The resulting sensors are capable of exhibiting high sensitivity in the x and y axes, though generally significantly lower sensitivity in the z-axis.
In view of the above, there is a need for three-axis inertial sensors capable of exhibiting high sensitivities in the x-axis, y-axis, and z-axis in a small die size, using a structure and fabrication process that is simple and capable of high yields.